Code division multiple access (CDMA) is quickly becoming the predominant worldwide wireless technology, due to its high subscriber capacity and its immunity to interference. As a result, consumer demand for high performance wireless communication devices has never been higher and there is no indication that this trend will decrease in the near future.
When designing high performance CDMA wireless communication devices, the issue of cross modulation between transmitter leakage at the receiver input and a jammer signal presents difficult challenges. Depending on the transmit power of the CDMA wireless communications device and the jammer signal power, a cross modulation spectrum may be produced at the output of the low noise amplifier (LNA) at the front-end of the receiver. This cross modulation spectrum may overlap the desired signal, and as a result, degrade the sensitivity of the receiver.
One way to reduce cross modulation is to increase the linearity of the LNA, or increase its IIP3 (3rd order Input intercept point). The problem with this approach is that any increase in linearity of the LNA results in a corresponding increase in power consumption, thereby creating an undesirable drain on the battery. This may result in reduced standby and talk time on the CDMA wireless communications device.
Another approach is to reduce the transmitter leakage at the input to the receiver. This may be achieved by either decreasing the transmit power of the wireless communications device, or increasing the transmit signal rejection of the duplexer. The reduction in transmit power may not be desirable, or even possible, when the user on the wireless communications device is traveling along the cellular boundaries. Increasing the duplexer's transmit signal rejection, on the other hand, is quite possible, and may be implemented with internal filtering. The drawback of this approach is that the in-band insertion loss of the duplexer is increased, which may needlessly degrade the sensitivity of the receiver when the transmit power of the wireless communications device is low. Accordingly, there is a need in the art for an adaptive filter that varies the transmit signal rejection as a function of the transmit power level of the wireless communications device and the jammer signal power.